Fluoropolymer fluid overfill probe with infrared optical signal

ABSTRACT

An optically-based overfill detection probe is provided that has a prism with a substantially quadrilateral cross-sectional shape, such that internal reflection angles are less than ninety degrees. The prism is made of a fluoropolymer, preferably TEFLON® PFA, and uses an infrared optical signal. The prism material has a high resistance to chemical solvents, has high signal transmission in the infrared (IR) wavelength range, is less prone to condensation and frost than plastic or glass, and enables the production of prisms at significantly less cost than glass. The substantially quadrilateral cross-sectional shape of the prism, in itself, has the advantage of allowing angles of incidence much less than ninety degrees on internal prism surfaces, thus reducing signal loss from surface irregularities. The probe also includes a two-piece housing structure that compresses an outwardly-extending portion of the prism material to firmly hold the prism in place, and force the material into sealing contact with an inner surface of the housing. The housing has an elongate portion which encompasses the prism, and which extends toward the fluid, preventing any light which escapes the prism from being externally reflected back to the photodetector of the probe.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fluid transfer control apparatus and,particularly, to the use of optically-based overfill probes fordetecting when fluid being transferred into a container has reached apredetermined level.

2. Description of the Related Art

In the art of fluid transfer control, particularly as it applies to thepetroleum industry, one of the more common control devices is anoverfill sensor for determining when the fluid being transferred into acontainer, such as a petroleum tanker compartment, has reached apredetermined level. An output signal from such a probe indicates whenthe fluid has reached the predetermined level, and may be used as anindication by a fluid transfer controller to discontinue fluid flow intothe container. In this way, overfilling of the container, which isparticularly hazardous when dealing with flammable liquids such asgasoline, can be avoided.

One type of overfill probe which is particularly common in thepetrochemical industry makes use of an optical signal which is coupledinto a medium having a relatively high index of refraction, such as aglass or non-opaque plastic. This medium is specially-shaped andcommonly referred to as a “prism.” The prism is shaped to cause internalreflection of the optical signal when surrounded by air. The shape ofthe prism and the direction at which the optical signal is coupled intothe prism is such that the reflection of the optical signal within theprism redirects the signal toward a photodetector. This photodetectorgenerates an output signal which indicates that the optical signal isbeing detected.

A schematic illustration of this prior art probe design is shown in FIG.1. In the plane of the optical signal path, the prism 10 has atriangular cross section. The optical signal is generated by lightsource 12. When the prism 10 is surrounded by air, the optical signal(indicated by the arrows in FIG. 1) is reflected at two interfacesbetween the prism material and the surrounding air, and redirectedtoward photodetector 14. The photodetector 14 generates an electricaloutput signal which indicates that the optical signal is being detected.

As shown in FIG. 1, the prior art prism 10 uses a forty-five degreeincidence angle (relative to normal) for each of the reflections of theoptical signal within the prism 10. This prism 10 has the triangularcross section shown, and light source 12 and photodetector 14 areoriented in the same direction along the same surface of the prism 10.When in use, the prism is part of a probe which is located within afluid container, usually near the top of the container. When the fluidin the container rises high enough to contact a prism surface at alocation where the optical signal is incident, the forty-five degreeangle is no longer sufficient to provide internal reflection of theoptical signal at that interface. This is because the prism/airinterface becomes a prism/fluid interface, and the fluid has an index ofrefraction much closer to the prism material than does air. According toSnell's law of refraction, (well-known in the art of optical design) theforty-five degree angle of incidence of the optical signal now resultsin the transmission of the optical signal through the interface due tothe similarity of the relative indices of refraction. As a result, thesignal is no longer detected by photodetector 14, and the correspondingchange in the photodetector output signal is used to discontinue loadingof the container.

One of the problems encountered with a prior art probe such as thatshown in FIG. 1 is related to the operational temperature range of theprobe. When the probe is used in cold ambient temperatures (common for apetroleum tanker truck which has the probe within one of its tankercompartments and which delivers fuel in regions having relatively coldclimates), is that condensation, or even frost, may form on the externalsurfaces of the prism. If sufficient condensation forms on the prismwhen the fluid level in the container is below that at which it shouldbe detected by the probe, the condensation may nonetheless causetransmission of a significant portion of the optical signal through thesurface of the prism. This portion of the signal then goes undetected bythe photodetector. If the signal loss is high enough, the signaldetected by the photodetector (and indicated by the photodetector outputsignal) may be below the detection threshold used to indicate when thefluid in the container has reached the probe level. As a result, a falseoverfill signal may result which prevents fluid from being loaded intothe container, despite the fact that the container may be empty.

In the past, one of the solutions to the condensation problem has beento increase the sensitivity of the photodetector so that it is activatedby smaller amounts of reflected light. However, this also makes theprobe more sensitive to inadvertent reflections from surfaces within thecontainer. When the prism is in contact with the fluid, the light fromthe light source can pass through the fluid, be reflected off areflective surface within the container, and find its way back to thephotodetector. If the reflected signal is strong enough, this can resultin a dangerous overfill situation, as the contact of the prism by thefluid goes undetected, and the container continues to be filled to thepoint of overflowing.

SUMMARY OF THE INVENTION

The improved overfill probe of the present invention makes use of anoptical signal in the infrared (IR) range, generated from an IR source,such as an diode having an output in the IR range. The optical signal iscoupled into a first medium of fluoropolymer, in the preferredembodiment TEFLON Perfluoro Alkoxy (TEFLON PFA), although otherfluoropolymers may also be used. TEFLON PFA is manufactured by, andTEFLON® is a registered trademark of, E.I. du Pont de Nemours & Co.,Inc. The prism has a particular shape which results in the internalreflection of the IR signal when the reflecting surfaces are contactedby a second medium (e.g. air) having an index of refractionsignificantly lower than that of the prism material. The reflection ofthe optical signal is toward a photodetector of the probe, which detectsthe optical signal and generates an output signal in response thereto.

The probe is located in a fluid container, such as a compartment of apetroleum tanker truck, with the prism positioned such that it iscontacted by fluid in the container when the fluid is at a predeterminedfluid level. The optical signal from the IR source is coupled into theprism and, while the fluid level is below the predetermined level (i.e.while the probe is surrounded by the second medium), the optical signalis reflected by at least one interface between the prism and the secondmedium. The optical signal is ultimately directed toward thephotodetector through internal reflection within the prism. As thecontainer is filled with liquid, the fluid level rises toward the prism.When the fluid reaches the prism, the new optical interface formed bythe prism and the fluid allows transmission of the optical signalthrough the interface. Without the reflection of the optical signal, thesignal is no longer detected by the photodetector. As a result, theoutput signal of the photodetector changes, indicating that the opticalsignal is no longer detected, and the change can be used by a fluidtransfer controller to discontinue fluid transfer into the container,thereby preventing overfilling.

In addition to the unique material of the probe prism of the presentinvention, the prism is also a unique shape. In particular, the prismhas a cross-sectional shape which is preferably substantially aquadrilateral. This cross-sectional shape results in the light sourceand photodetector not being oriented in the same direction, but alsoprovides a higher angle of incidence (relative to normal) of the opticalsignal on the internally reflective surfaces of the prism. There istherefore less chance of optical leakage through the reflective surfacesof the prism due to localized surface irregularities (i.e. due to thesurface not being perfectly smooth on a microscopic level, as may resultfrom moisture or frost) and better overall signal performance.

The present invention also includes a two-piece probe housing whichscrews together to enclose the probe components. The tightening togetherof the housing portions compresses an outwardly-extending portion of theprism material, firmly holding it in place. The compression of thismaterial also causes it to flow outwardly, sealing it against an innersurface of the housing. The housing also includes a lower portion whichis roughly cylindrical with cutouts along its surface. The cutouts allowair to escape as fluid rises within the cylindrical portion. Thesurfaces of the lower portion surrounding the cutouts prevent light fromthe light source from being inadvertently reflected off a reflectivesurface within the container back to the photodetector when fluid in thecontainer is in contact with the prism.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the optical prism of a typical priorart fluid overfill probe.

FIG. 2 is a schematic depiction of the optical prism of a fluid overfillprobe according to the present invention.

FIG. 3 is a cross-sectional, exploded view of a fluid overfill probeaccording to the present invention.

FIG. 4 is a cross-sectional assembled view of the fluid overfill probeof FIG. 3.

FIG. 5 is a cross-sectional view of a fluid container within which islocated a fluid overfill probe according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The overfill probe of the present invention uses an optical signal whichis coupled into a prism 16 by a light source 18. FIG. 2 is across-sectional schematic illustration of the probe 16 and light source18. In the preferred embodiment, the prism 16 is made of one of severaldifferent types of fluoropolymer. In the preferred embodiment, TEFLON®PFA (perfluoro alkoxy) is used, which is manufactured by E.I. du Pont DeNemours & Co., Inc., while in an alternative embodiment the prism isTEFLON FEP (fluorinated ethylene propylene). It will be understood bythose skilled in the art that other materials (particularly otherfluoropolymers) may be used which provide similar qualities to thosementioned above. Hereinafter, the term “TEFLON” is used to refergenerally to the fluoropolymer materials which are particularlywell-suited for the present invention.

The TEFLON material used has the advantage that it is chemically inertto most industrial chemicals and solvents. As such, the prism can comein contact with any of a wide array of different chemical liquids andgases without being damaged. This allows the probe to be used in a widevariety of different liquid detection applications. TEFLON prisms alsohave a distinct advantage over glass or plastic prisms when used inprobes for overfill detection in the petrochemical industry, such as inthe compartments of tanker trucks. This advantage lies in the fact thatTEFLON is much less prone to optical signal loss due to condensation orfrost on the surface of the probe. The TEFLON prism is alsosignificantly less expensive to produce than a glass prism. Furthermore,while inhibiting transmission of visible light, a TEFLON probe is moretransmissible to light in the infrared (IR) band than is a glass probe.Light emitting diodes (LEDs) which emit light at an IR wavelength have aparticularly good optical power output, one which, for the sameelectrical power input, is typically higher than that of LEDs at thevisible wavelengths commonly used with glass or plastic prisms.Therefore, in the preferred embodiment, light source 18 is an infraredLED.

Referring again to FIG. 2, the cross-sectional shape of the prism of thepresent invention is quadrilateral, as opposed to the triangularcross-sectional shape of prior art prisms. As such, the LED 18 and thephotodetector 20 are oriented in different directions along differentcross-sectional surfaces. The “surfaces” 24A, 24B, 26A, 26B of FIG. 2are referred to as “cross-sectional” surfaces since, in actuality, thetop of the prism and the bottom of the prism each can be, and in thepreferred embodiment each is, a single conical surface. However, it willbe understood by those skilled in the art that the important geometry ofthe prism is the roughly planar cross section within which the bulk ofthe optical signal travels through the prism. The relevant geometry ofthe prism is therefore described herein with reference to the crosssection of FIG. 2. Obviously, neither the top nor bottom of the prismtherefore need to be conical, and could be pyramidal or hemispherical,for example.

In the preferred embodiment, the FIG. 2 cross section of prism 16 issymmetrical about center line 22. This symmetry simplifies manufactureof the prism and determination of its relevant dimensions. However, suchsymmetry is not necessary, provided the relevant cross-sectional angularrelationship between LED 18, photodetector 20 and the reflective prismsurfaces is maintained. In the preferred embodiment, the shape of theprism may be defined by two angles within the cross sectional planeshown in FIG. 2. The first angle, α, is the angle from normal at whichthe light rays are incident upon the surfaces 26A, 26B of the prism. Thesecond angle, β, is one half the internal angle between surfaces 24A and24B (i.e. the angle between either of surfaces 24A, 24B and center line22). Given either of these angles, the other may be determined by thefollowing relationship:

β=2(90°−α)

This relationship also depends on LED 18 and photodetector 20 beingoriented so as to transmit and receive light, respectively, in adirection normal to surfaces 24A, 24B. Variations of the presentinvention might use different orientations for LED 18 and photodetector20, and thereby alter the geometry of the prism but, for maximum opticalcoupling through surfaces 24A, 24B and ease of manufacture, the presentinvention uses a perpendicular orientation of the LED 18 andphotodetector 20 relative to the surfaces 24A, 24B.

In the preferred embodiment, the use of TEFLON PFA requires that α isgreater than approximately 47.79°. This limitation results from anapplication of Snell's law of refraction, given the manufacturer'slisted index of refraction of TEFLON PFA. The 47.79° limit isapproximate since the listed index of refraction is based on the ASTMtesting standard, and is not specifically matched to the IR wavelengthof LED 18. In the preferred embodiment, the angle α=55° and, therefore,the angle β=70°.

While the quadrilateral cross-sectional shape of the prism 16 isparticularly well-suited to the TEFLON materials preferred with thepresent invention, it is noted that this geometry may also havebeneficial effects for other materials, including the glass and plasticsof conventional prisms. For example, the quadrilateral shape allows ahigher angle of incidence (relative to normal) on the reflectivesurfaces than a triangular cross section, while still having aperpendicular orientation of the LED and photodetector relative to thesurfaces through which they transmit and receive light, respectively.

A preferred embodiment of the present invention makes use of a two-pieceprobe housing, as in FIGS. 3 and 4. FIG. 3 is a cross-sectional explodedview of a probe according to the present invention, in which thecomponents of the probe are located within a probe housing comprisingupper portion 28 and lower portion 30. As shown in FIG. 3, the prism 16includes a portion which extends outward so as to form mounting seat 32.When the prism is located with the housing, seat 32 sits flush againstlip 34 of lower housing portion 30. This keeps the prism centered withinopening 36 of lower portion 30, into which fluid may rise when the probeis positioned within a container being filled. Hereinafter, the entireTEFLON structure (including the outwardly-extending portion as well asthe optical portion having the substantially quadrilateral shape) willbe referred to as the prism 16.

On the upper side of prism 16 is a cavity 38 which receives optical seat40. Optical seat 40 is roughly the shape of a truncated cone, althoughthe surface of the seat 40 facing prism 16 is itself conical, and shapedto fit snugly against the upper surface of prism 16. Optical seat 40 hastwo bores through its body, one of which receives LED 18, and one ofwhich receives photodetector 20. The bores in seat 40 are sized tosnugly accommodate the LED 18 and photodetector 20, respectively, andare angled so as to orient each of the LED and photodetector normal tothe surface of prism 16 which it faces. Thus, when the optical seat 40is fitted within cavity 38, and LED 18 and photodetector 20 are mountedwithin their respective bores, light is transmitted from the LED 18 atan angle of 90° relative to the prism surface through which it isdirected, and light is received by the photodetector 20 at an angle of90° relative to the prism surface through which it is received.

Also shown in FIG. 3 is circuit board 42 to which the LED 18 andphotodetector 20 are electrically connected. The circuit board 42 isshown in cutaway so as not to obscure the features of upper housingportion 28. Those skilled in the art will understand that the circuitboard 42 resides within the hollow body of upper portion 28, andcontains electrical circuitry used in the generation of optical signalsby LED 18 and the processing of optical signals detected byphotodetector 20.

When the probe is assembled, screw threads 44 of upper housing portion28 are meshed with complementary screw threads 46 of lower housingportion 30, such that the two portions are screwed tightly together. Anannular lip of optical seat 40, and an annular top surface 50 of theprism (which are flush with each other when assembled) are contacted bythe annular lip 52 of upper housing portion 28. When the two housingportions are completely screwed together, the separation between upperportion lip 50 and lower portion lip 34 is less than theoriginally-fabricated distance between annular lip 52 and mounting seat32. The housing surfaces 34, 52 therefore compress the prism material inthis annular region. Since TEFLON (both PFA TEFLON and FEP TEFLON) flowsunder pressure, the prism material flows outward toward the innersurface 54 of the upper housing portion 28. This forcing of the materialagainst the inside of the housing provides the probe with a good seal tohelp prevent liquid or gas from seeping into the probe housing from thecontainer.

The probe of FIG. 3 is depicted in its assembled form in FIG. 4. The topof the probe and circuit board 42 are shown in cutaway to provide moredetail of the probe components. As shown, the portion of the probematerial between the lip 50 and mounting seat 32 is forced against theinside of the upper housing portion 28 forming the aforementioned seal.The opening 36 in lower housing portion 30 is generally cylindrical,being open at the bottom, but includes two cutouts 56 (only one beingshown in the cross section of FIG. 4, the other existing opposite it inthe housing body). These cutouts allow air to escape while fluid entersthe opening 36 as it rises in the container within which it is mounted.

The part of the lower housing portion which is not cut out extends to alength at which, if a reflective surface was in contact with the end ofthe lower housing portion, and fluid was in contact with the prism, nolight from the LED would be reflected from the reflective surface backto the photodetector. This prevents an unexpected reflective surface(such as a shiny metal container surface or a white piece of materialfloating in the container) from causing a false detection of the opticalsignal by the photodetector. This part of the lower portion also reducesthe effects of ambient light on the probe, and protects the prism fromscratching.

FIG. 5 shows a probe according to the present invention within a fluidcontainer 62. This figure is not to scale, but is used to demonstratethe functionality of the overfill probe. As the container 62 is filledwith a liquid 58 from nozzle 60, the liquid level rises toward theprobe. When the liquid 58 contacts the probe prism, the optical signalof the probe is no longer reflected within the body of the prism, andescapes through the prism surface into the fluid. The resulting changein the output of the probe photodetector is then used as an indicationthat the container is full, and the flow of fluid 58 into the containeris discontinued.

While the invention has been shown and described with regard to apreferred embodiment thereof, those skilled in the art will recognizethat various changes in form and detail may be made herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A fluid overfill detection probe comprising: alight source which emits an optical signal having a center wavelength inthe infrared range; a photodetector which detects the optical signal;and a prism into which the optical signal is coupled by the lightsource, the prism having a substantially quadrilateral cross section andcomprising a fluoropolymer material which results in an internalreflection of the optical signal from the light source toward thephotodetector when a surface of the prism at which said reflectionoccurs is not contacted by a fluid being detected, and which does notresult in said internal reflection when said prism surface is contactedby said fluid.
 2. A fluid overfill detection probe according to claim 1wherein the prism material comprises TEFLON PFA.
 3. A fluid overfilldetection probe according to claim 1 wherein the prism materialcomprises TEFLON FEP.
 4. A fluid overfill detection probe according toclaim 1 wherein the light source is a light-emitting diode (LED).
 5. Afluid overfill detection probe according to claim 1 further comprising ahousing having a plurality of portions which, when connected together,enclose the light source, photodetector and prism.
 6. A fluid overfilldetection probe according to claim 5 wherein the housing portions havecomplementary screw threads which allow them to be screwed together. 7.A fluid overfill detection probe according to claim 1 further comprisingan optical seat within which the light source and photodetector aresecured, the optical seat being securely located relative to the prismsuch that the light source and photodetector are in predeterminedorientations relative to the prism.
 8. A fluid overfill detection probeaccording to claim 1 wherein an angle of incidence upon said prismsurface is significantly greater than forty-five degrees.
 9. A fluidoverfill detection probe according to claim 1 wherein an angle ofincidence upon said prism surface is greater than fifty degrees.
 10. Amethod of constructing an optically-based overfill probe, the methodcomprising: forming a prism of the probe from a fluoropolymer into ashape having a cross section which is substantially quadrilateral;providing an infrared light source which couples light into the prismsubstantially normal to a first cross-sectional prism surface, such thatthe light from the light source is internally reflected off at least oneinternal surface of the prism when the prism is surrounded by air; andproviding a photodetector which detects infrared light from a directionsubstantially normal to a second cross-sectional prism surface, theorientation of the first prism surface and the second prism surfacebeing such that said internally reflected light is detected by thephotodetector.